inlet to move the liquid into the pump. Atmospheric pressure, acting on the surface of the liquid, then provides the necessary pressure to move the liquid into the pump.
The principle of suction force, or suction lift, as applied to reciprocating pumps, is shown in figure 6-6. In diagram A, the piston cylinder is open at both the top and bottom, so the liquid level at A and B is the same. In diagram B, the cylinder is closed at the bottom. A piston has been inserted and partly withdrawn, thus creating a partial vacuum. When the foot valve (check valve) at the bottom of the cylinder opens (diagram C), as a result of the lower pressure in the cylinder, the liquid rises up into the cylinder, which causes the liquid level in the well to drop. Assuming the liquid is water and there is a perfect vacuum below the piston, atmospheric pressure pushes water up into the cylinder to a height of 34 feet, even though the piston may be raised higher than 34 feet.
You must understand that the preceding example is for the theoretical condition of a perfect vacuum. In practice, leakage between the piston and the cylinder, friction (fluid) in piping, and gases dissolved in the liquid limit the suction lift of a pump to a height of approximately 22 feet, as shown in diagram D of figure 6-6.
When a pump is pumping certain liquids, such as hot water, oil, or gasoline, some of the liquid vaporizes because of the vacuum on the suction side of the pump. The pump may become vapor bound and reduce the possible suction lift; this is called "cavitation."
Figure 6-6. - Diagrams showing the principles of suction force.
The suction force principle applies to other types of pumps, as well as to the reciprocating type, though to a lesser degree and in a different manner. The centrifugal, the propeller, and the rotary pumps all use suction force to a certain extent. Here, a partial vacuum can be produced by the revolving mechanisms instead of by the reciprocating plunger. Also, centrifugal pumps are not self-priming because they do not pump air. Their casing must be flooded before they can function. In the eductor (jet pump), flow is maintained by the suction force created by a jet of water, compressed air, or steam passing through a nozzle at high velocity. These principles are explained later in this chapter.
Every pump is equipped with devices for controlling the direction of flow, the volume of flow, and the operating pressure of the pump. A device that performs one or more of these control functions is called a VALVE.
A valve that permits liquid flow in only one direction is classified as a CHECK VALVE. In most cases, check valves open and close automatically; that is, they are kept closed or seated by spring tension or by the force of gravity until the liquid pressure above or below the valve overcomes the spring or gravity resistance and causes the valve to open. Check valves of this type are used with centrifugal pumps to control the suction and discharge of the liquid in the pump end at the proper time automatically. Figure 6-7 shows a vertical check valve. In this case, the valve is kept seated by its own weight or the force of gravity. If desired, it could also be kept closed by a spring.
Another type of valve in pump systems is the STOP VALVE. Stop valves are usually opened or
Figure 6-7. - Vertical check valve.Continue Reading